Gravity is a force between two objects with mass.
It pulls things together. You have mass, and the
Earth has mass, so gravity tries to pull you and
the Earth together. The gravitational force is
much bigger for more massive objects. The Earth
has a lot of mass, which is why it can hold you
down. You can't feel the gravity attracting you
to smaller objects, like your house or other
people. That's because these things aren't
anywhere near as massive as the entire
Earth.

OK, so you know what gravity is.
But how does it actually pull stuff together?
Actually, scientists aren't sure. There is a
theory that gravity is made up of little particles
called gravitons, just like light is made up of
little particles called photons. But we've never
seen a graviton! So we're not actually sure they
exist.